That looks about right. You could get a ballpark number by taking what the tag says that you would typically use in a year and mulitply by (1-energy factor). Really, the total has the energy factor already included (so a portion of it would be inculded twice with this basic calc), but it would get you a rough number.
Another way to do it would be to look at the annual operating cost on the tag and mulitply by (1-energy factor). For instance, a 50-gal Rheem Imperial is listed at 0.95 EF and has an annual operating cost of $492 (assuming 10.65 cents/KWH). $492*0.05 = $24.60/year
There will be additional standby losses from piping cooling down between uses or if the WH is located in an unconditioned space. However, the losses in the pipes will be the same for any type of WH used (insulate the pipes that you can get to).
Also, if the WH is in a conditioned space, the EF becomes 1.00 during the heating season as any of the "wasted" heat goes into warming the house, so the actual loss from the tank will actually be less than the $24 or so, especially if you live somewhere with a long heating season.
Electric units do very well in terms of standby loss as they have a heavy insulated blanket arond them. Gas models tend to have lower EF as the insulation is usually thinner as well as what is lost up the flue. However, gas may still be cheaper to run for a lot of people and gas also has a higher recovery rate. For my location, electricty is low and gas is somewhat high. The cost per BTU/KWH would be similar, but since I don't have other gas appliances, gas would cost me more as there is about a $13/month service charge for gas and that would kill any savings that I might get from a gas WH.
If you run the numbers, you'll find it very difficult to justify a tankless unit based on what is saved on standby losses (even if you were starting with a low EF gas unit). People also tend to use more hot water when they have a tankless since they have an "unlimited" supply. Tankless units are good if you don't have much space or have a large tub to fill (or a shower that is more like a car wash than a shower ).